1. Field of the Invention
This invention relates to an automatic plate bending system using high frequency induction heating, and more specifically, to one useful for application to the bending of a steel plate having complicated curved surfaces, such as an outer panel of a ship hull.
2. Description of the Prior Art
The outer panel of a ship hull is composed of a steel plate about 10 to 30 mm thick with a complicated undevelopable curved surface which reduces propulsion resistance for efficient navigation in the water. To form this curved outer panel, a processing method generally called line heating has been known for long. This method heats the surface of a steel plate locally by means of a gas burner or the like, to cause the extraplane angular deformation or intraplane shrinkage deformation of the steel plate due to plastic distortion, and skillfully combines these deformations to obtain the desired shape. This method is used at many shipyards.
FIG. 1 is an explanation drawing conceptually showing an earlier technology concerned with a method for bending a steel plate to serve as an outer panel of a ship hull. FIG. 2 is a front view showing a wooden pattern for use in the bending in a state in which it is mounted on the steel plate. As shown in both drawings, according to the earlier technology, many (10 in the drawing) wooden patterns 1 following frame lines of the outer panel of the ship hull (lines extending along frame materials for the outer panel at positions where the frame materials are attached; the same will hold in the following description) as target shapes are mounted on a steel plate 2. Then, an operator compares the shapes of each wooden pattern 1 and the steel plate 2 by visual observation, and considers differences between their shapes, e.g., the clearance between the wooden pattern 1 and the steel plate 2. Based on this consideration, the operator studies what position to heat in order to bring the steel plate 2 close to the target shape. As a result, the operator determines each heating position (heating point). Concretely, the wooden pattern 1 is rolled along the frame line of the steel plate 2 in a vertical plane (the same plane as in FIG. 2). The points of contact of the wooden pattern 1 with the steel plate 2 during the rolling motion are watched to determine the heating points in consideration of the clearance between the wooden pattern 1 and the steel plate 2 in each state.
Then, it is considered how to connect the respective heating points together in order to make the steel plate 2 similar to the target shape. Based on this consideration, a heating line is determined. As shown in FIG. 3, heating lines 3 that have been determined are marked on the surface of the steel plate 2 with chalk or the like, and the steel plate 2 is heated with a gas burner along the heating lines 3.
With the earlier technology as described above, the steel plate 2 is heated with a gas burner by the operator along the heating lines 3 determined by the operator's sense based on many years of experience. As a result, a predetermined curved surface is obtained. Acquiring the ability to determine the heating lines 3 rationally is said to require more than about 5 years of experience. This has posed the problems of the aging and shortage of experienced technicians. The bending procedure also takes a large amount of time for incidental operations, such as the production, mounting and removal of the wooden pattern 1 for the steel plate 2, thus lengthening the entire operating time. Besides, the heating operation using a gas burner itself becomes heavily muscular activity in a hot, humid harsh environment involving the occurrence of steam associated with the evaporation of cooling water. Hence, a demand is growing for the advent of a device which realizes the automation of the plate bending operation.
To solve the problem of the shortage of experienced technicians and reduce the operating time, it is necessary to improve, theorize and automate the bending operation while taking into consideration know-how that operators acquired through experience.
Generally, bending of a plate material such as a steel plate is performed using a press or the like. To process the plate material into a complicated shape which is hard to form with a press, hot bending by a gas burner is used. The operation using a gas burner causes the problem of a deteriorated work environment due to noise, heat and combustion gases. Recently, therefore, high frequency induction heating has been studied. High frequency induction heating produces eddy currents in a member to be heated, e.g., a steel plate, by the action of electromagnetic induction, and applies heat by utilizing an eddy-current loss. Thus, a high frequency heating coil is required for high frequency induction heating.
FIG. 4 shows an example of a high frequency induction heater for heating a flat plate-shaped member to be heated, such as a steel plate 1, from above. A high frequency heating coil 02 is provided opposite the steel plate 1 via a clearance .DELTA.t so as to be movable by a moving device 04 in the direction of an arrow A. The clearance .DELTA.t is about 5 mm. The high frequency heating coil 02 is secured to a lower end of a bar-shaped support arm 05 via a disk portion 03, and the support arm 05 is supported by a guide portion 04a of the moving device 04 so as to be movable vertically. Thus, the high frequency heating coil 02 moves linearly in a vertical direction integrally with the support arm 05. The moving device 04 has its moving speed controlled by a moving speed controller 06, and moves horizontally linearly along a guide rail 07. In the drawing, the reference numeral 08 denotes a matching transformer, and the numeral 09 designates a high frequency power source. To achieve desired uniform heating with such a high frequency induction heater, it is vital to keep the clearance .DELTA.t between the high frequency heating coil 02 and the steel plate 1 constant. This is because a heat input to the steel plate 1 is determined simply by the clearance .DELTA.t as a parameter along with an electric current supplied to the high frequency heating coil 02, its frequency, and the moving speed of the high frequency heating coil 02.
High frequency induction heating thus requires that the clearance .DELTA.t between the high frequency heating coil 02 and the steel plate 1 be kept constant. To meet this requirement, the high frequency induction heater according to the earlier technology has a laser sensor provided near the high frequency heating coil 02, measures the distance between the high frequency heating coil 02 and the steel plate 1 by the laser sensor, and extends or contracts the support arm 05 to keep the clearance .DELTA.t between the high frequency heating coil 02 and the steel plate 1 constant. However, the laser sensor is vulnerable to high temperatures or steam. Thus, it is difficult to protect the laser sensor, for example, from radiant heat generated when the temperature of the steel plate 1 rises to 800.degree. C., or from steam produced when the heated steel plate 1 is cooled with water. There is also the problem that laser light is disturbed by steam, and measurement errors will result.
Hot bending of the steel plate involves various forms of heating, including line heating for heating in a linear form, spot heating for heating predetermined spots in a circular form, weaving heating for heating in a zigzag form, and pine needle heating for heating in a triangular form.
To accommodate various forms of heating mentioned above, various coils adapted to the forms of heating are made ready for use, and a coil may be changed to agree with the form of heating. That is, an attachment type coil may be used. For such an attachment type, however, many coils in agreement with the forms of heating must be prepared, and coil replacement is required each time the form of heating is changed. This presents with the problems of boosted equipment cost and decreased operating efficiency.